Seamless steel pipes selection principles of for pressure pipelines

　　Seamless steel pipes selection principles of steel pipe for pressure pipelines,A pressure pipe is a tubular device used to transport gas or liquid under a certain pressure. Its scope is stipulated as the gas, liquefied gas, steam medium with the maximum working pressure greater than or equal to 0.1MPa, or the liquid medium with the highest working temperature higher than or equal to the standard boiling point, which is flammable, explosive, toxic or corrosive,and the pipe with nominal diameter greater than 25mm.Principles for selection of commonly used steel pipe materials for pressure pipelines.

　　At present, seamless steel pipe (DN15-600) is the most widely used pipe in petrochemical production equipment.

　　　(1) Preferred pipe

　　When selecting pipe materials, generally metal materials should be considered first, and non-metal materials should be considered when metal materials are not applicable. Steel pipes are preferred for metal materials, and non-ferrous metal materials will be considered later. Among steel pipes, carbon steel should be considered first, and stainless steel should be used when it is not applicable. When considering carbon steel materials, first consider welded steel pipes, and then choose seamless steel pipes when not applicable.

　　(2) The influence of medium pressure

　　The higher the pressure of the conveying medium, the thicker the wall thickness of the pipe, and the higher the requirements for the pipe material.

　　When the medium pressure is above 1.6MPa, seamless steel pipes or non-ferrous metal pipes can be used. When the pressure is very high, such as in the production of synthetic ammonia, urea and methanol, the pressure of some pipes is as high as 32MPa. Generally, 20# or 15CrMo high-pressure seamless steel pipes are used. The pipes on the vacuum equipment and the oxygen pipes when the pressure is greater than 10MPa generally use copper pipes and brass pipes.

　　When the medium pressure is below 1.6MPa, welded steel pipes, cast iron pipes or non-metallic pipes can be considered. However, the pressure of the cast iron pipe withstand the medium shall not be greater than 1.0MPa. The medium pressure that the non-metallic pipe can withstand is related to the variety of non-metallic materials, such as unplasticized polyvinyl chloride pipes, whose working pressure is less than or equal to 1.6MPa; reinforced polypropylene pipes, whose working pressure is less than or equal to 1.0MPa; ABS pipes, whose working pressure is less than or equal to 1.0MPa. Less than or equal to 0.6MPa.

　　For water pipes, when the water pressure is below 1.0MPa, the welded steel pipe of Q235A is usually used; when the water pressure is greater than 2.5MPa, the seamless steel pipe of 20# is generally used.

　　(3) The influence of medium temperature

　　Tubes of different materials are suitable for different temperature ranges. For hydrogen with a pressure of 1.0MPa, when the hydrogen temperature is less than 350°C, 20# seamless steel pipes are generally used, and when the hydrogen temperature is in the range of 351~400°C, 15CrMo or 12CrMo seamless steel pipes are generally used.

　　(4) The influence of the chemical properties of the medium

　　Conveying different media, using different pipes. Some mediums are neutral, and generally do not require high materials, so ordinary carbon steel pipes can be selected; some mediums are acidic or alkaline, and acid-resistant or alkali-resistant pipes must be selected. Strong acid and strong base and weak acid and weak base have different requirements for the use of pipes. The same acid or alkali has different requirements for the material of the pipe with different concentrations. For transporting water and water vapor, a pipe made of carbon steel is sufficient. For example, in the urea plant, the pipes for conveying carbon dioxide generally use stainless steel pipes, because carbon dioxide meets water to form carbonic acid, which has a corrosive effect on general steel pipes. For example, carbon steel pipes can be used for fuming sulfuric acid, but carbon steel pipes should not be used for dilute sulfuric acid. Because dilute sulfuric acid and carbon steel can react chemically and corrode carbon steel, duralumin pipes can be used.

　　(5) The influence of the function of the tube itself

　　In addition to the function of transporting the medium, some pipes also have the function of shock absorption, coefficient of thermal expansion and contraction, and the function of frequent movement under working conditions, such as civil liquefied petroleum gas, oxygen, and acetylene gas. Whereas, high-pressure steel wire braided rubber hoses are often used for pipes, instead of hard steel pipes that are inconvenient to move.

　　(6) The influence of pressure drop

　　After the initial selection of the pipe material, the pressure drop of the pipe must be calculated to determine the inner diameter of the pipe. Through the calculation of pressure drop, see whether the selected material meets the requirements. Especially when initially selecting plastic pipes, more attention should be paid to the review of pressure drop.

　　In the calculation of pressure pipelines, in engineering design, material balance, energy balance and equipment calculation are generally carried out according to the production scale, and the material flow rate is initially determined. And refer to the relevant information, assume a material flow rate, calculate the inner diameter of the pipe, check the manual or standard, and select the standard pipe. The inner diameter of the standard pipe usually selected should be equal to or slightly larger than the calculated pipe inner diameter. Then calculate the pressure drop of the pipeline.

　　#20 Seamless Steel Pipes: The 20 in #20 steel refers to the carbon content, and the carbon content is 0.2%, which is a low-carbon steel. The yield strength is 245Mpa, and the tensile strength is 410Mpa.

　　When designing pressure piping, after the pipe diameter is calculated and determined, the material of the pipe must be selected. The use of commonly used pipe materials for pressure pipes is determined according to the operating conditions of the conveyed medium (such as pressure, temperature) and its medium characteristics under these conditions. Improper selection of materials will cause waste or bury hidden dangers of accidents.